Synchronous extraction circuits for enhanced piezoelectric energy harvesting generally require dedicated electronic units for switching operations, but additional effects of higher start voltage threshold, more cost and energy consumption are introduced. As an alternative solution with less electronic components and lower voltage threshold, this article presents a novel synchronous switching circuit with a new self-adaptive peak-detection mechanical switch design which is composed of two auxiliary oscillators of low resonant frequency, a piezoelectric generator and two soft spring electrodes between them. In contrast with previous mechanical switch solutions for fixed displacement amplitude or frequency, this design can automatically trace the displacement amplitude and conduct the synchronous switching operations so that the performance can be greatly improved. Automatic peak-detection and switching ability for different vibration amplitude and frequencies is experimentally demonstrated by this circuit with the proposed self-adaptive mechanical switches. A power enhancement of 375% has been achieved by this new circuit in comparison with the standard approach. Further discussions have been performed to detail the structure characteristics while the possible application of the piezoelectric generator with the self-adaptive mechanical switch in vehicle monitoring is suggested.